1. Field of the Invention
The present invention relates to a numerically controlled machining method for controlling a machine tool having a numerical control apparatus so as to machine a workpiece, and more particularly to a numerically controlled machining method capable of easily cutting off any portion of the workpiece left uncut by a primary cutter.
2. Prior Art
Numerically controlled machine tools (hereinafter referred to as "NC machine tools") controlled by numerical control apparatus (hereinafter referred to as "NC apparatus") are given a tool position with respect to a workpiece under the command of numerical information, and are controlled by the results of arithmetic operations effected by the NC apparatus to enable the tool to machine the workpiece. NC machine tools can machine workpieces into complex configurations with ease and high precision at a high rate of production.
An NC machine tool is generally constructed as shown in FIG. 1 of the accompanying drawings. The NC machine tool basically comprises an NC apparatus 120 for computing numerical information commands supplied from an external source through an input terminal 110 and a machine tool 130 controlled by the results of arithmetic operations effected in the NC apparatus 120. The NC apparatus 120 is composed of an input unit 121 to which external commands are supplied, an arithmetic unit 122 for computing the commands delivered from the input unit 121, a memory unit 123 for storing the results of arithmetic operations in the arithmetic unit 122 and the commands from the input unit 121, and other information, a control unit 124 for controlling the arithmetic operations in the arithmetic unit 122, and an output unit 125 for issuing commands or computed values such as the results of the arithmetic operations in the arithmetic unit 122.
The machine tool 130 has a tool 131 attached to a tool holder 132 mounted on the chuck of a spindle 133 which is rotatable by a spindle motor 134 driven by signals issued from the output unit 125 of the NC apparatus 120. A workpiece 140 to be machined by the machine tool 130 is fixed by jigs or the like to a table 135 of the machine tool 130. The machine tool 130 also includes a leadscrew 136 for moving the table 135 in the direction of the X axis. The leadscrew 136 is driven by an X-axis motor 138 through a gear box 137, the X-axis motor 138 being driven by signals from the output unit 125 of the NC apparatus 120. The machine tool 130 also includes mechanisms (not shown), identical to the X-axis drive motor 138 and the leadscrew 136 for moving the table 135 in the direction of Y- and Z-axes. These mechanisms are also driven by signals supplied from the NC apparatus 120.
FIG. 2 schematically shows a lathe, as an example of an NC machine tool controlled by a numerically controlled machining method (hereinafter referred to as an "NC machining method").
A cylindrical workpiece 12 to be machined is held by a chuck 10 rotatable about a Z-axis and has an end supported by a tailstock center 14a of a tail stock 14. A turret 16 holds a tool 18 such as a cutter for cutting the workpiece 12. For cutting the workpiece 12, the turret 16 is moved in the direction of the Z-axis to cause the cutter 18 to cut the workpiece 12.
FIG. 3 fragmentarily illustrates, on an enlarged scale, the workpiece 12 to be machined by a primary cutter 20 into a final profile indicated by the dot-and-dash line. However, the primary cutter 20 has a finite cutting angle l and fails to cut the workpiece 12 to the final contour, including planes perpendicular to the Z-axis, unless another suitable cutter is used, since the primary cutter 20 can only cut the workpiece 12 to a rough profile within the desired final shape shown in FIG. 3. Accordingly, after the workpiece 12 has been machined by the primary cutter 20, there are left uncut portions 22, 24, shown hatched in FIG. 4, which need to be removed. For removal of such uncut portions 22, 24, it has been customary for a programmer to measure their dimensions and to effect a separate processing step to cut off the portions 22, 24. Therefore, the conventional process for removing such unwanted uncut portions has been tedious and time-consuming.